Content overview
Candidates study the following topics:
1 States of matter
2 Atoms, elements and compounds
3 Stoichiometry
4 Electrochemistry
5 Chemical energetics
6 Chemical reactions
7 Acids, bases and salts
8 The Periodic Table
9 Metals
10 Chemistry of the environment
11 Organic chemistry
12 Experimental techniques and chemical analysis
Stoichiometry: The Mathematics of Chemical Reactions
Stoichiometry is a branch of chemistry that deals with the quantitative relationships between the
reactants and products in a chemical reaction. By using stoichiometry, chemists can predict the
amounts of substances consumed and produced in a reaction, ensuring accurate and efficient
chemical processes.
The Basics of Stoichiometry
1. Chemical Equations:
o A balanced chemical equation is the starting point for stoichiometric calculations.
It represents the chemical reaction, showing the reactants (substances that react)
and the products (substances formed).
o Example: 2H2+O2→2H2O\text{2H}_2 + \text{O}_2 \rightarrow \text{2H}_2\
text{O}2H2+O2→2H2O This equation tells us that two molecules of hydrogen
gas react with one molecule of oxygen gas to produce two molecules of water.
2. Mole Concept:
o The mole is a fundamental unit in stoichiometry, representing 6.022×10236.022 \
times 10^{23}6.022×1023 particles of a substance (Avogadro's number).
o Molar mass (grams per mole) relates the mass of a substance to the number of
moles.
3. Law of Conservation of Mass:
o This law states that matter cannot be created or destroyed in a chemical reaction.
The total mass of reactants equals the total mass of products.
Stoichiometric Calculations
Stoichiometric calculations involve determining the amounts of reactants or products based on a
balanced chemical equation. The steps typically include:
1. Write and Balance the Equation:
o Ensure the chemical equation is balanced, with equal numbers of atoms for each
element on both sides.
2. Convert Quantities to Moles:
o Use the molar mass to convert grams to moles (if given mass) or other units to
moles.
3. Use Mole Ratios:
o Mole ratios, derived from the coefficients in the balanced equation, relate the
quantities of reactants and products.
4. Calculate Desired Quantities:
o Convert moles back to grams, liters, or other units as needed.
Types of Stoichiometric Problems
1. Mass-to-Mass Calculations:
o Example: How many grams of water (H2O\text{H}_2\text{O}H2O) are produced
when 4 grams of hydrogen gas (H2\text{H}_2H2) react with oxygen?
Balanced Equation: 2H2+O2→2H2O\text{2H}_2 + \text{O}_2 \
rightarrow \text{2H}_2\text{O}2H2+O2→2H2O
Steps:
1. Calculate moles of H2\text{H}_2H2: 4 g2 g/mol=2 mol\frac{4 \, \
text{g}}{2 \, \text{g/mol}} = 2 \, \text{mol}2g/mol4g=2mol.
2. Use mole ratio: 2 mol H2→2 mol H2O2 \, \text{mol} \, \text{H}_2
\rightarrow 2 \, \text{mol} \, \text{H}_2\text{O}2molH2
→2molH2O.
3. Calculate mass of H2O\text{H}_2\text{O}H2O: 2 mol×18
g/mol=36 g2 \, \text{mol} \times 18 \, \text{g/mol} = 36 \, \
text{g}2mol×18g/mol=36g.
2. Limiting Reactant:
o Determines which reactant will run out first, limiting the amount of product
formed.
o Example: Given 5 moles of H2\text{H}_2H2 and 3 moles of O2\text{O}_2O2,
which is the limiting reactant?
Mole ratio: 2 H2:1 O22 \, \text{H}_2 : 1 \, \text{O}_22H2:1O2.
Required O2\text{O}_2O2 for 5 moles of H2\text{H}_2H2: 52=2.5 mol \
frac{5}{2} = 2.5 \, \text{mol}25=2.5mol.
Since 3 moles of O2\text{O}_2O2 are available, H2\text{H}_2H2 is the
limiting reactant.
3. Theoretical Yield:
o The maximum amount of product that can be formed, based on the limiting
reactant.
o Actual yield (measured experimentally) is often less than theoretical yield due to
inefficiencies.
4. Percent Yield:
o Percent yield measures reaction efficiency:
Percent Yield=(Actual YieldTheoretical Yield)×100\text{Percent Yield} = \left( \
frac{\text{Actual Yield}}{\text{Theoretical Yield}} \right) \times
100Percent Yield=(Theoretical YieldActual Yield)×100
Applications of Stoichiometry
1. Chemical Manufacturing:
o Ensures optimal use of raw materials and minimizes waste in industrial processes.
2. Pharmaceuticals:
o Determines precise quantities of chemicals for drug formulation.
3. Environmental Science:
o Assesses pollutant emissions and their neutralization in reactions.
4. Space Exploration:
o Calculates fuel requirements and oxygen generation for astronauts.
Advanced Stoichiometry
1. Solutions and Concentrations:
o Reactions in solutions use molarity (moles of solute per liter of solution) for
stoichiometric calculations.
2. Gaseous Reactions:
o Use the ideal gas law (PV=nRTPV = nRTPV=nRT) to relate the volume,
pressure, and temperature of gases.
3. Energy Changes:
o Combines stoichiometry with thermodynamics to calculate energy released or
absorbed in reactions.
Conclusion
Stoichiometry is the mathematical framework that connects the macroscopic quantities of
materials used and produced in chemical reactions with their microscopic atomic and molecular
interactions. Its principles are vital for science, industry, and everyday life, enabling the efficient
and sustainable use of resources.
